JP-A-2007-180892, known as one of the related prior art techniques, discloses a camera comprising a face detecting circuit, a gamma compensation controller and a picture signal processing circuit, wherein the face detecting circuit detects a face from an inputted picture signal and calculates a brightness change estimation value for the facial area including the detected face; the gamma compensation controller determines a luminance scale characteristic on the basis of the brightness change estimation value calculated by the face detecting circuit; and the picture signal processing circuit performs gamma compensation for the inputted picture signal on the basis of the luminance scale characteristic determined by the gamma compensation controller.
With the popularization of digital cameras capable of taking motion pictures as well as still pictures, it has come to be possible for every body to take good pictures. People are the most favorite objects that are most often photographed. Therefore, such cameras are increasing on the market as are equipped with face recognition and optimization functions which automatically recognize the face of a person and optimizes the camera setting according to the recognized face.
Such a face recognizing function as equipped in the cameras mentioned above, compares the inputted picture signal with the pattern stored previously in the memory in the camera, and detects the area in the imaging surface in the camera on which the face is focused (this area is hereafter referred to as “facial field”). Data gathered from within the detected facial field include the position of the facial field, exposure metering value, color, etc. When the detected facial field is actually photographed, proper procedures is automatically performed on the basis of the gathered data. The proper procedures meant here includes, for example, providing proper brightness on the facial field, focusing of the facial field preferentially, controlling white balance such that the facial field is properly colored, and adjusting the chrominance signal such that natural skin color can be displayed.
The automatic compensation of brightness in the facial field to maintain it at a proper level of brightness, which was enumerated above as the first of the proper procedures mentioned above, is strongly required by users. When a person is photographed, it often happens that the picture is taken against the sun, that is, the difference in brightness between the object and the background is very large. In such a case, the brightness of the object, i.e. person, cannot usually be maintained at a proper level since the brightness of the background adversely affects that of the object. It might be possible in this case to properly adjust the brightness of the object manually, but such manual compensation is awkward and especially difficult for ordinary users to do.
There is known a conventional technique for automatically compensating for the brightness of the facial field such that it is maintained at a proper level of brightness. According to the conventional technique, exposure control value (for proper control of iris, shutter and AGC, i.e. automatic gain control, circuit) is varied in accordance with the target exposure amount calculated on the basis of the exposure metering value in the facial field. However, since the exposure control value is that which controls the brightness of the entire imaged picture, the brightness of the facial field can indeed be maintained at a proper level, but the brightness of the background becomes improper so that overexposure or underexposure may occur in the background.
There is also known a conventional technique for automatically compensating for the brightness of the facial field to maintain it at a proper level of brightness, and preventing such overexposure and underexposure. According to this technique, gamma compensation control is varied in accordance with the target exposure amount calculated on the basis of the exposure metering value in the facial field. The gamma compensation control is to change the luminance scale characteristic of an imaged picture. And by properly controlling the gamma compensation, the brightness of the facial field and the luminance scale of the background can be made compatible with each other.
A technique for changing the luminance scale characteristic through the control of gamma compensation in accordance with the data collected in the facial field by the face recognition function, is disclosed in, for example, in the above mentioned JP-A-2007-180892. According to JP-A-2007-180892, even if there is difference in brightness within the facial field, proper brightness can be reproduced by calculating the difference in brightness within the facial field and then by using the luminance scale characteristic in accordance with the calculated brightness difference. According to this method, however, only the difference in brightness within the facial field is regarded as the criterion for gamma compensation, the reproducibility of the luminance scale of the background brightness is not so much taken into consideration.
As described in the foregoing, in order to make compatible the brightness of the face of a person and the luminance scale of the background in photographing the person, it is necessary to properly control exposure and gamma compensation on the basis of the data collected within the facial field and to select such exposure control value and luminance scale characteristic as correspond to a picture to be imaged.
Conventional face recognition and optimization function has not taken it into consideration to reproduce the luminance scale of the background brightness since only difference in brightness within the facial field is regarded as the criterion for gamma compensation.